a) Would it remove the offending backup file by renaming it locally and running a new (incremental) backup?
b) Would it remove the offending backup file to delete prior backups and do a fresh full backup with the file of concern properly encrypted?
after initial posting, I found this from Don_Wright:
Think of each backup as a point-in-time copy of the files at that moment. When UrBackup manages storage by deleting older backups, it will also notice the files that are not needed for later backups (because of changes or deletion since then) and purge the no-longer-needed copies from storage. So yes, things can be lost, but not generally anything currently present on the sets being backed up.
Note also the special case of files that are present on several sets. These will only be deleted after all backups that refer to that version of the file have been deleted. If everyone has the 2021 Policy Manual but April is also keeping the 2019 version she worked on, UrBackup will have to keep both versions in storage, but only one copy of each. The 2020 Manual is no longer saved on any set, so as soon as the last old backup from 2020 is deleted, it will vanish as well.